Science Briefs

Studying Molecular Properties for Aircraft Design

December 1995

Early launches of the Space Shuttle had problems with thermal insulation tiles. These tiles are important because vehicles reentering the atmosphere at high speed are heated by collisions with air molecules. Part of the energy from these collisions goes into heating the surface of the vehicle and the remainder goes into internal energy of the air molecules so that there is also a layer of very hot gas around the vehicle. Although this effect is included in the design process, it is difficult to make accurate predictions of the temperatures that can be reached because we do not yet completely understand all of the
contributing molecular processes. The temperature reached at the
surface of the vehicle and the temperature gradient in the
surrounding air are controlled by the rate at which the excited
molecules near the hot surface can transfer their energy to
cooler molecules which are further away. While it is easy to
measure these rates in undisturbed gases, measurements are quite difficult in the quickly changing environment close to a hypersonic vehicle, so values required for aircraft design remain poorly known.

Winifred Huo (NASA/Ames) and Sheldon Green (NASA/GISS) have recently shown that it is possible to predict these molecular rates from the basic laws of physics by solving equations of quantum theory. Although these equations are very complicated, recent increases in available computing power have permitted much progress. Huo and Green used supercomputers at NASA's Numerical Aerodynamic Simulation facility to predict energy transfer in nitrogen, the main constituent of air, at room temperature, finding excellent agreement with experimental values, which are well known for such accessible conditions. These results suggest that similar calculations can provide the high temperature and nonequilibrium values which are required for airplane design.